eBay return - how to get eBay involved?

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I have been sent a fake item on eBay. Photos/description of genuine OEM part, but cheap unbranded part sent instead.

I raised a return dispute but he's not interested. It says I have 8 days to work it out with him before eBay intervene. Is there any way to speed that up? Should I dispute direct with PayPal?

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Is it OK to leave negative feedback in the meantime? Or should I wait until the whole thing is resolved when eBay intervene?

I've been on eBay over 10 years and this will be the first negative feedback I leave. I've been sent fake items before, but never had a seller be so nasty and argumentative when they're in the wrong!